Growing hydroponically has many benefits including increased yield, less space, no soil, etc. But there is one significant disadvantage: you have to know water chemistry. Learn how!

Water chemistry involves many factors. Two very important factors are totally dissolved solids (TDS) and pH. People sometimes express TDS as electrical conductivity (EC), because electrical conductivity is the physical measurement your device takes, and TDS can be inferred from this measurement because all dissolved salts contribute to the electrical conductivity of water.

While TDS, or EC, is simple enough because it measures the amount of things you add to water, plus what is already present in the water (salt, magnesium, calcium, etc.), pH is more complicated. Here, we will attempt to explain the very misunderstood and pesky problem of pH in tap water slowly increasing over time.

Everyone knows that things are more soluble in hot water, like salt or sugar for example. But did you know the trend is the opposite for gases?

This means the gases are more soluble in cold water than in hot water. There are only two soluble gases that are important to a grower: oxygen and carbon dioxide. Roots need some dissolved oxygen to stay alive, which means you shouldn’t let the water get too hot.

How does dissolved carbon dioxide affect your reservoir?

After CO2 dissolves in water, it forms carbonic acid. This reaction is reversible, meaning CO2 basically goes in and out of water as it pleases. In this case, the temperature of the water is the main influencing factor we need to consider.

CO2 dissolves in tap water, in often copious amounts, due to bubbling and frothing in the plumbing. Normally, this tap water comes out cold, usually at around 50 F. As it slowly warms, carbonic acid turns into carbon dioxide which evaporates along with other gases. Less acid means the pH goes up. It is often a good idea to let tap water sit for some time to let chlorine dissipate, but not many people realize how much that can also change their pH.

If tap water’s pH started out at around 7 out of the faucet, once it has time to fully equilibrate with its surroundings, it may begin to show its true colors with a pH of around 8. It may be useful to do your own experiment to figure out how much the pH changes after your water adjusts to room temperature. Take a pH measurement right when it comes out, and then again after an hour or more. Be sure to leave the container open to the air as it sits.

The pH of your nutrient solution will also drift mildly once it comes in contact with your plants. As the roots take up nutrients, they shift the pH up and down, and you might also have some effects due to carbon dioxide exchange with the plants and the air.

As long as the pH doesn’t ever get below 5.5 or above 6.8, you should be alright.

Do not chase after the ideal pH everyday by constantly adding “pH Up” or “pH Down”. This will probably cause excess TDS, which can prevent roots from absorbing anything at all.

It is important to always remember that quickly adjusting pH can kill your plants.

Ariana Marisol is a contributing staff writer for REALfarmacy.com. She is an avid nature enthusiast, gardener, photographer, writer, hiker, dreamer, and lover of all things sustainable, wild, and free. Ariana strives to bring people closer to their true source, Mother Nature. She graduated The Evergreen State College with an undergraduate degree focusing on Sustainable Design and Environmental Science.